Acquired immunodeficiency syndrome (AIDS) B-cell lymphomas occur in individuals in the 30-year age group with a history of homosexuality, bisexuality, and intravenous drug use. Thirty percent of these lymphomas are, histologically, Burkitt's type and contain the Epstein-Barr virus (EBV) genome. The other B-cell lymphomas are diffuse, large cell type which lack EBV association. Based on these findings, the interaction of EBV, possible other human viruses, and HIV in B-cell lymphomas was investigated. B-cell lymphomas associated with EBV developed in an HIV-virus- positive and antibody-positive AIDS patient six months after significant increase in EBV-early antigen antibody (EA) titers were observed. This suggested that EBV may lead to polyclonal proliferation of B-cells, one of which may undergo transformation. During the course of these investigations, a novel human B lymphotropic virus (HBLV) was isolated from peripheral blood lymphocytes from two AIDS patients with B-cell lymphomas. Later on, HBLV was also isolated from four patients with angioimmunoblastic lymphoadenopathy, immunoblastic lymphoma, and lymphocytic leukemia. HBLV is a new herpesvirus which is genetically and immunologically distinct from known herpesviruses.